Editor-in-chief Donald Kennedy and news editor Colin Norman tasked their staff to list the most challenging questions in science today and then winnow the number to 25.

To make the cut, questions had to be "tough enough and challenging enough and inviting to people who read them" to inspire readers to think about "what the solution[s] might be," Kennedy said.

Highlights from the top 25:

What Is the Universe Made Of?

In recent decades, scientists have discovered that the ordinary matter that makes up stars, planets, even human beings, accounts for only 5 percent of everything in the universe. The rest belongs to dark matter and dark energy, phenomena that scientists are just now learning about.

What is dark matter made of and where does it reside? What is dark energy? Researchers hope to find answers.

What Is the Biological Basis of Consciousness?

In the 17th century, French philosopher and mathematician René Descartes declared that mind and body are entirely separate, leaving the debate over the nature of consciousness to other philosophers.

Today scientists are challenging that notion with a view that consciousness arises from the properties and the organization of neurons in the brain. Experimental work to unravel those properties and processes has only just begun.

"If the results don't provide a blinding insight into how consciousness arises from tangles of neurons, they should at least refine the next round of questions," Greg Miller writes in the Science special issue.